The present invention relates to a nanocrystalline alloy having excellent pulse attenuation characteristics and a method of producing it. The present invention further relates to a choke coil utilizing the nanocrystalline alloy and a noise filter composed of the choke coil.
As a material for a magnetic core of a common-mode choke coil used in a noise filter, a high permeability material having excellent high-frequency properties such as ferrite, amorphous alloy, etc. has been used. In addition, JP-B-4-4393 discloses that an Fe-based fine crystalline alloy (nanocrystalline alloy) is suitable as a material for such a magnetic core because it has a high permeability and low core loss.
The material for a common-mode choke coil used in a noise filter (line filter) is further required to have not only a high permeability but also excellent pulse attenuation characteristics for preventing disordered operating of an apparatus due to high-voltage pulse noise caused by thunder, etc.
However, since the ferrite material, which has been conventionally used, is low in saturation magnetic flux density, it easily reaches a magnetically-saturated state. This results in a problem that a small-sized core made of the ferrite material cannot meet the above requirements and such a core shows only insufficient efficiency. Therefore, a large-sized core is necessary for obtaining a high efficiency when ferrite is used as the core material.
An Fe-based amorphous alloy has a high saturation magnetic flux density and shows, with respect to a high-voltage pulse noise, more excellent attenuation characteristics than those shown by the ferrite material. However, since the permeability of the Fe-based amorphous alloy is lower than that of a Co-based amorphous alloy, it shows insufficient attenuation to a low-voltage noise. In addition, the Fe-based amorphous alloy has a remarkably large magnetostriction. This invites further problems such as alteration in its properties caused by a resonance which may occur at a certain frequency due to the magnetostriction, and occurrence of beat in case of including audio frequency component.
On the other hand, a Co-based amorphous alloy shows a large attenuation to low-voltage noise due to its high permeability. However, its saturation magnetic flux density is lower than 1 T or less and it shows poor attenuation to high-voltage pulse noise as compared with an Fe-based amorphous alloy. Further, the Co-based amorphous alloy of a high permeability largely changes, in particular under environment of a high surrounding temperature, its properties with the passage of time, this resulting in lack of reliance.
As described above, the Fe-based fine crystalline alloy (nanocrystalline alloy) disclosed in JP-B-4-4393 has been known to have a high permeability and low core loss. However, the conventional Fe-based fine crystalline alloy is usually subjected to heat treatment while applying a magnetic field in the transverse direction (width direction) of a thin alloy ribbon in order to improve its pulse attenuation characteristics, because it cannot be provided with sufficient attenuation characteristics when subjected to heat treatment without applying any magnetic field. However, in this heat treatment in a magnetic field, it is required to make a core material (a thin alloy ribbon) magnetically saturated by the applied magnetic field. For meeting this requirement, a magnetic field of 1000 A/m or more is necessary to be applied because of a large demagnetizing field. Therefore, the heat treatment in a magnetic field is costly due to a great deal of consumed electrical power. In addition, it is low in productivity due to the necessity to keep the core to be treated at an accurate location because the application direction of magnetic field must be maintained at a constant direction. As described above, when the Fe-based fine crystalline alloy is subjected to heat treatment without applying any magnetic field, it cannot be provided with a sufficient attenuation to a high-voltage pulse noise. Therefore, if a nanocrystalline alloy having pulse attenuation characteristics comparable to or more excellent than that of a nanocrystalline alloy produced by heat treatment in a magnetic field can be produced by without applying any magnetic field, its industrial advantage would be greatly significant.